1. Field of the Invention
The invention relates to a process and a device for joining of workpiece elements by means of an energy beam, in particular a laser beam, making use of a filler material which is added in the form of a wire by means of a wire feeding device, whereby the wire is also used as a scanning element.
2. Description of the Related Art
The connecting of workpiece elements by laser beam welding or laser beam soldering making use of a filler wire is becoming of increasing importance in particular in the automobile manufacturing sector. One important aspect of this joining technique is the precise adjustment of the position of the filler wire relative to the laser beam. The adjustment is relatively difficult, however, because it must be very precise, the working laser beam is small and not visible to the human eye, and the structure of known laser beam devices renders observation of the joining process difficult. The position of the laser beam and the filler wire relative to the joint of the workpiece elements which are to be connected to one another must however be precisely maintained during the advance movement of the guidance machine used for this purpose. While the distance between the laser focusing device and the joint with the beam geometries conventionally used allows for relative large deviations to be tolerated, the lateral position of the laser beam to the joint and the distance between the filler wire and the joint, and its lateral position in relation to it during joining must be precisely maintained during the joining process.
Devices are known which have a regulated filler wire advance with fixed established wire feed positions.
In addition, devices are known which with the aid of sensors determine the position of the joint of the workpiece elements which are to be connected to one another, and the laser focusing device is guided in this position by actuators, together with the filler wire feeding device which is firmly connected to it during the joining process. Such a device is described, for example, in DE 44 15 035 C2. The position of the joint in this case can be determined only spatially in advance in relation to the joining process, since the joint is no longer identifiable at the immediate welding or soldering point. Such devices accordingly operate relatively imprecisely, despite their elaborate and expensive structure.
In addition to this, devices are known in which the entire joint contour is travelled over in a cycle carried out before the joining, and, with the aid of sensors, the actual current position of the joint contour is determined, in order for it to be travelled over in a second work sequence and for the workpiece elements to be joined during this sequence. The disadvantage here, however, is the additional time required for the preceding travel over the joint contour, and the fact that a thermal distortion during the jointing process must remain unconsidered.
From DE 44 12 093 A1, a device is known for controlling a laser welding machine, which comprises a reversible feed motor for the focusing head and a feed device secured to it for a filler material, with a guidance size voltage being imposed between the filler material and its feed device on the one hand and the workpiece on the other, the pick-up of which is connected to two parallel comparators, one of which is connected to a reference value voltage for a maximum value of a control range, and the other with a reference value voltage for a minimum value of the range. Both comparators take effect in this situation on a power controller, which in turn determines the desired direction of rotation and the operation of the feed motor. The filler material can be provided in any desired form, in particular in the form of a wire made of electrically conductive material. With this device the wire is therefore also used as an electrical scanning element.
Finally, devices are known which, by means of mechanical scanners and flexible securing, adjust the distance from the workpiece of a point in front of, behind, or next to the laser beam (related to the course of the welding or soldering seam; see for example DE 196 15 069 A1). Such scanners are however subject to mechanical wear. In addition, the angle position of the device in relation to the surface of the workpiece has a substantial influence on the precision of the joining process, since the mechanical scanner is supported on a point in front of, behind, or next to the immediate joint position.
The object of the present invention is to provide a process and device of the type referred to in the preamble, which will provide optimum guidance of the wire, especially in the direction of the axis of the energy beam.
The object is resolved in respect of the process according to the invention in that the filler wire is used as a mechanical scanning element, whereby the wire tip melting in the energy beam is guided in non-positive fit along the surface of the workpiece.
The invention is accordingly based on the idea of using the filler wire as a mechanical scanning element. By contrast with the conventional mechanical scanners, the filler wire is practically free of any wear, since it is continually melted in the energy beam. The filler wire scans the contour of the joint defined by the workpiece elements which are to be joined, by means of the tip of the wire in the edge of the immediate weld or solder position. The energy beam is for preference a laser beam. It is also conceivable, however, that the invention can also be used with electron beam joining.
If the wire guiding device is arranged in a displaceable manner in the direction of the axis of the energy beam and the wire top melted in the energy beam is guided in non-positive fit along the surface of the workpiece, an interval between the melting wire tip and the workpiece surface will automatically be established, which will ensure the optimum melting of the wire at the edge of the energy beam, in contact with the workpiece surface. In a lateral direction in relation to the welding or soldering seam which is to be created, the adjustment of the wire to the energy beam can be effected preferably by means of a visible auxiliary laser beam (a pilot laser).
The guidance of the energy beam in the direction of the interval, i.e. the guidance of the energy beam focus in the direction of the axis of the beam, can, according to a preferred embodiment of the process according to the invention, be effected in such a way that the device for focusing the energy beam is guided with a fixed distance from the workpiece surface. As an alternative the device for focusing the energy beam can also automatically be moved together with the wire feeding device. The latter arrangement is similar to known mechanical scanners, but with the difference that the reference point lies on the edge of the molten bath, i.e. on the edge of the welding or soldering position at that particular moment.
A further advantageous embodiment of the process according to the invention is provided in that, with the workpiece elements which are to be joined, which form a connecting joint with at least one lateral guide surface, the wire tip which is to be melted is guided in non-positive fit along the lateral guide surface. Such lateral guidance is possible, for example, in the creation of a flange weld or fillet weld. The laser beam and the wire guiding device can in this situation, being coupled to one another, be guided relative to the lateral guide surface by means of lateral displacement and/or a rotational movement.
It is further to advantage if the distance between the device for focusing the energy beam and the workpiece surface is determined and the joining process terminated in the event of a predetermined distance being exceeded or undercut. Thus, for example, the end of a welding seat can be identified at the end of a piece of sheet metal by a substantial change in the distance, and the joining process being ended accordingly.
According to a further advantageous embodiment of the process, provision is made for the contact pressure force of the wire tip being compared with a reference contact pressure force and the joining process being terminated if a predetermined reference contact pressure is exceeded or undercut. On the basis of the comparison of the actual contact pressure force of the wire tip with a reference contact pressure force, a conclusion can be drawn about erroneous guidance, excessive workpiece tolerances, poor joint preparation, deficient energy beam performance, and/or deficient advance movement of the guiding machine concerned. This enables the occurrence of qualitatively deficient welding or soldering connections to be identified in good time and for the appropriate correction to be carried out and an overload to the system avoided.
With regard to the device, the objective according to the invention is resolved in that the wire guiding device is mounted on bearings in the direction of the axis of the energy beam in such a way that the wire tip melting in the energy beam is guided in non-positive fit along the workpiece surface. The non-positive guidance of the wire tip can in this situation be effected simply by the force of gravity, a spring, pneumatic or hydraulic drives with specific pressure, electric motors with specific torque, or by means of sensors, in particular moment of force sensors, and actuators guided and regulated in accordance with these sensors. The regulation of the force by actuators with sensors which measure the contact pressure force of the wire as a scanning element allows for the exclusion of the influence of the inherent weight of the wire guiding device in the event of a change of its spatial orientation.